Bird egg

Bird eggs are laid by female birds. They come in many different sizes and numbers. Some birds, like condors, lay just one egg. Others, like the grey partridge, can lay up to seventeen eggs at a time. The number of eggs a bird lays can change depending on where they live.

Birds sometimes lay eggs that are not fertilized. This means they won't hatch into baby birds. These eggs are often called wind-eggs. Pet bird owners might find these eggs in their bird's nest.

Eggs are an important part of how birds reproduce and continue their species. Learning about bird eggs helps us understand more about the lives and habits of different birds.

Anatomy

Bird eggs are special little packages that help baby birds grow. Inside each egg, there is an embryo, which is the tiny developing chick. The egg has a special liquid called amniotic fluid inside a membrane called the amnion. This fluid protects the embryo from bumps. Another part called the allantois helps the embryo get oxygen and take care of its waste.

The yellow part of the egg, called the vitellus or yolk, gives the baby bird food and important nutrients. Around the embryo is a clear liquid called albumen, made of proteins. The egg is wrapped in a shell with tiny holes that let air in while keeping out anything harmful.

Colors

Bird eggs are often white because of the calcium carbonate in their shells. Many birds, especially passerines, lay colored eggs. Pigments like biliverdin and protoporphyrin make eggs green, blue, red, or brown. Some birds, like Charadriiformes, sandgrouse, and nightjars, have white eggs to blend in with the ground where they nest.

Birds that build nests in trees often have blue or green eggs. Birds that nest on or near the ground usually have speckled eggs. Egg color is influenced by genetics and is passed from the mother. In some species, like the common guillemot, each female's eggs look different, helping them find their own eggs in busy nesting areas.

Shell

Bird eggshells come in many types. For example, cormorant eggs are rough and chalky, tinamou eggs are shiny, duck eggs are oily and waterproof, and cassowary eggs are heavily pitted. Tiny pores in a bird eggshell help the embryo breathe. A domestic hen's egg has about 7,500 of these pores.

Shape

Bird eggs come in many different shapes. Some, like those of the little bee-eater, are almost round. Others, like those of the common murre, are pointy and cone-shaped. The familiar chicken egg is in between these two shapes.

Scientists used to think the shape of an egg was set as it passed through a bird's body. Now we know the shape is set before the shell hardens. Birds that nest on cliffs often have pointy eggs so they don’t roll away. Birds that nest in holes usually have more round eggs. The shape of an egg can also be linked to how the bird flies. Stronger fliers tend to have more elliptical eggs.

Size

Bird eggs come in many different sizes, depending on the size of the bird. The tiniest bird, the bee hummingbird, lays eggs that weigh about half a gram. The huge ostrich can lay eggs that weigh as much as 1.5 kilograms. Some birds, like kiwis, lay very large eggs — up to 20% of the mother bird’s weight. These big eggs help kiwi chicks hatch and find their own food a few days later.

Number

The number of eggs a bird lays in one nesting is called its clutch size. Most birds lay a small, steady number of eggs each time. Some birds will lay a new egg if one is lost, while others stop laying if there are too many eggs. Scientists group birds into two types. Determinate layers lay a set number of eggs. Indeterminate layers may change how many eggs they lay. Birds that usually lay just a few eggs are often indeterminate layers.

Predation

Main article: Egg predation

Many animals eat bird eggs. Raccoons, skunks, mink, otters, gulls, crows, and foxes often eat eggs from birds like the black oystercatcher. Small animals such as stoats and weasels may take eggs from ducks. Large animals like mongooses, vultures, and hyenas can eat the big eggs of ostriches. Some snakes, like those in the genera Dasypeltis and Elachistodon, eat eggs as their main food.

Some birds lay their eggs in the nests of other birds. This tricks the host bird into raising their young. Birds such as cowbirds, honeyguides, and many cuckoos do this.

Evolutionary history, dinosaur eggs

Bird eggs have hard shells, which is special for animals today. This feature first came from a group of animals that included birds: dinosaurs. Because of this, a bird egg can also be called a dinosaur egg. Eggs with the modern structure, including special layers, appeared during the Jurassic period and are linked to fossils of the Maniraptora.